Translucent polycrystalline alumina (PCA) ceramic has made possible present-day high-pressure sodium (HPS) and ceramic metal halide lamps. The arc discharge vessels in these applications must be capable of withstanding the high temperatures and pressures generated in an operating lamp as well be resistant to chemical attack by the fill materials. In addition, the discharge vessels are typically required to have >92% total transmittance in the visible wavelength region from about 400 nm to about 700 nm in order to be useable in commercial lighting applications.
In HPS lamps, the discharge vessels are tubular, whereas for ceramic metal-halide lamps discharge vessels can range from a cylindrical shape to an approximately spherical shape (bulgy). Examples of these types of arc discharge vessels are given in European Patent Application No. 0 587 238 A1 and U.S. Pat. No. 5,936,351, respectively. The bulgy shape with its hemispherical ends yields a more uniform temperature distribution, resulting in reduced corrosion of the PCA by the lamp fills.
Because PCA is translucent and not transparent, the use of PCA is limited to non-focused-beam lamp applications. Birefringent grain scattering is a major source of in-line transmittance loss in regular, sintered PCA, and in-line transmittance generally increases with increasing grain size. Reducing the grain size of sinter-HIPed PCA to the submicron range (<1 micrometer) shifts the scattering mechanism thereby decreasing grain birefringent scattering. In the submicron region, the in-line transmittance actually increases with decreasing grain size. The high in-line transmittance and mechanical strength of submicron-grained PCA are of interest for focused-beam, short-arc lamps, that offer improved luminance, efficacy, and color rendition.
Magnesia (MgO) is typically required as a sintering aid in the manufacture of alumina discharge vessels in order to retard grain growth and facilitate grain boundary diffusion while pinning grain boundaries. Submicron alumina ceramics based on nano-sized starting powders generally require a higher level MgO to reach full density than larger-grained (10-30 microns) alumina based on micron-sized starting powders. This is because nano-sized powder requires a higher level of MgO dopant to cover the surface of the finer particles. Moreover, unlike the larger-grained alumina, the MgO-based dopants (e.g. ˜200-300 ppm) become completely dissolved in lattice and grain boundary region. As a result, high levels of color centers can form including a variety of single and double oxygen vacancies with one or two electrons. These color centers absorb light which result in a low total transmittance (˜78%) for MgO-doped, submicron-grained alumina discharge vessels despite their high in-line transmittance.